I3DL2 support

[Torinde]

I3DL2 is basically EAX2, specifying a per-buffer/source low-pass filter with an adjustable 5khz reference gain, and a reverb effect with parameters matching EAX2's reverb, which is also equivalent EFX's standard AL_EFFECT_REVERB (which is different from other types of reverb available at the time, that tended to be more abstract with less configurability; think like Freeverb). I thought I had a pdf about it, but I can't find it, and that site's download link doesn't seem to work. IDirectSoundFXI3DL2Reverb8 is Microsoft's implementation of the EAX2/I3DL2-style reverb, fitted to the DSound8 FX API.

Originally posted by @kcat in #36 (comment)

You mean the specs or was there something else in your mind?

Originally posted by @mirh in #36 (comment)

That, yeah. The structures and properties all seem to be identical to EAX 2, simply using the I3DL2 moniker instead of EAX, EAX2, or EAX20. They just use different GUIDs for the property sets. It might be possible to simply check for the I3DL2 GUIDs and substitute the EAX 2.0 ones when calling OpenAL with them. Unless there are apps that use both EAX2 and I3DL2 and they're expected to hold separate state, in which case OpenAL Soft would need to recognize the GUIDs and store the properties separately.

Originally posted by @kcat in #36 (comment)